1. Field of the Invention
This invention relates to a method and an apparatus for driving an optical recording medium for recording and/or reproducing various information for the optical recording medium. This application claims priority of Japanese Patent Application No. 2001-370497, filed on Dec. 4, 2001, the entirety of which is incorporated by reference herein.
2. Description of Related Art
Among optical recording media, typified by optical discs, a disc having pre-formed pits and used exclusively for replay, a disc which may be recorded and reproduced based on phase changes along a grooved structure or with the use of magneto-optical recording, and so forth, have so far been proposed. In the case of an optical recording medium driving device, such as an optical disc device employing an optical disc, which allows for recording and reproduction, a semiconductor laser, having a larger value of the maximum radiating light volume (maximum light output rating) is customarily used as a light source for an optical head. Meanwhile, a light source of a larger value of the maximum rating is usually not necessary for the following reason:    (1) When a semiconductor laser has a low output, stabilized oscillations can hardly be achieved, as a general rule, thus increasing the laser noise. Thus, in order to achieve the CNR (carrier to noise ratio) in reproducing the information, the laser light output needs to be set to a more or less large value which usually is 2 mW to 5 mW.    (2) In the case of a recordable optical recording medium, the information recording is by raising for example the temperature of the recording layer by a light beam converged on a recording surface thereof. In this case, if the two conditions of [no deterioration in recording signals with the reproducing optical power] and [stable recording with the recording optical power] are to be met, an output ratio of the optical power in recording to that in reproduction needs to be higher than a preset value. The maximum power of the recording light is routinely 5 to 20 times that for the reproducing light. In information recording at a rate higher than the standard rate, a higher output ratio is required.
For the above reasons, the maximum light output rating for a light source used for an optical head for recording and reproduction and for a light source used for an optical head for recording and/or reproducing plural sorts of the optical recording medium is usually 20 mW to 50 mW. For an optical recording medium for information recording at a higher rate of the order of eight times the standard rate, such as an optical disc of the so-called CD-R/RW format, the same rating is of the order of 100 mW.
However, the light source with a high maximum light output rating is difficult to fabricate, while power consumption of the light source is undesirably increased. If, in this consideration, a smaller value of the maximum light output rating is to be used, the laser noise at the time of reproduction is increased, such that replay characteristics achieved may not be optimum.
In replay-only DVD, such as DVD-ROM or DVD-Video, both being registered trademarks, an optical disc having two recording surfaces has already been put to practical utilization. For optical discs which are recordable and reproducible, proposals have already been made for multi-layered optical discs, having a multi-layered recording surface, such as two-layered or four-layered optical discs.
For these multi-layered optical recording media, the recording optical power and the reproducing optical power which are approximately 1.5 to 2 times as large as or even higher than that for the optical recording medium having a sole recording surface is needed. Thus, in a system adapted for coping with both an optical recording medium having a sole recording surface and an optical recording medium having plural recording surfaces, the ratio (multiplication factor) of the maximum recording optical power for a multi-layered optical disc to the reproducing optical power for a single layer optical disc, on simple approximation, amounts to two or thereabouts.
Moreover, the required recording optical power or reproducing optical power differs in case of a differing linear velocity of the optical recording medium relative to the light beam. That is, if the linear velocity of the optical recording medium relative to the light beam is increased, a larger recording optical power and a larger reproducing optical power are required.
Thus, with an increasing recording capacity, estimated in time to come, it may be premeditated that the dynamic range of the optical output of the light source has to be increased further.